Group transmission in a packet radio network

ABSTRACT

The invention relates to a method and apparatus for transmitting group messages in a packet radio network comprising at least one support node and a plurality of group members and at least one defined area. The support node knows the identity of group members in said area. The network comprises broadcast connections and point-to-point connections. The method according to the invention comprises: keeping track of the group members that have not received the group message and determining whether or not their number exceeds a predetermined threshold; in response to a positive determination in the previous step, transmitting the group message over a broadcast connection; and in response to a negative determination, transmitting the group message to the group members over a point-to-point connection.

FIELD OF THE INVENTION

The present invention relates in general to packet radio systems andmore particularly, the invention relates to a method and an arrangementfor broadcasting group messages in a packet radio network, preferably amobile packet radio network, such as GPRS.

BACKGROUND OF THE INVENTION

Mobile communication systems have been developed because it has beennecessary to be able to reach people even when they are not close to afixed telephone terminal. As the use of different data transmissionservices in offices has increased, different data services have alsobeen introduced into mobile communication systems. Portable computersenable efficient data processing everywhere the user moves. Mobilecommunication networks in turn provide the user with an efficient accessnetwork to actual data networks for mobile data transmission. In orderto realize this, different new data services are designed for existingand future mobile communication networks. Digital mobile communicationsystems, such as the pan-European mobile communication system GSM(Global System for Mobile Communication), support particularly wellmobile data transmission.

General Packet Radio Service (GPRS) is a new service in the GSM system,and it is one the items of the standardization work of the GSM phase 2+in ETSI (European Telecommunication Standard Institute). The GPRSoperational environment consists of one or more sub-network serviceareas, which are interconnected by a GPRS backbone network. Asub-network comprises a number of packet data service nodes, which arereferred to as GPRS support nodes (or agents) in this context, eachpacket data service node being connected to a GSM mobile communicationnetwork in such a manner that it is capable of providing a packet dataservice for mobile data terminal equipment via several base stations,i.e. cells. The intermediate mobile communication network providescircuit switched or packet switched data transmission between a supportnode and mobile data terminal equipment. Different sub-networks areconnected to an external data network, such as a public switched packetdata network PSPDN. The GPRS service thus produces packet datatransmission between mobile data terminal equipment and external datanetworks, a GSM network acting as an access network. One aspect of theGPRS service network is that it operates almost independently of the GSMnetwork. One of the requirements set for the GPRS service is that itmust operate together with external PSPDNs of different types, forinstance with the Internet or X.25 networks. In other words, the GPRSservice and a GSM network should be capable of serving all users,irrespective of the type of data networks they want to register in viathe GSM network. This means that the GSM network and the GPRS servicehave to support and process different network addressing methods anddata packet formats. This data packet processing also comprises therouting the packets in a packet radio network. In addition, the usersshould be able to roam from a GPRS home network to an external GPRSnetwork, the operator of which has a backbone network that may support aprotocol (for instance CLNP) that is different from the one supported bythe home network (for instance X.25).

Referring to FIG. 1, a typical arrangement in a GPRS network will now bedescribed. It should be understood that the architecture of GPRS systemsis not as advanced as that of GSM systems. Therefore, all GPRS termsshould be interpreted as being descriptive rather than limiting terms. Atypical mobile station constituting mobile data terminal equipmentconsists of a mobile station MS in a mobile communication network, and aportable computer PC connected to the data interface of said mobilestation MS. The mobile station MS may be for instance a Nokia 2110,which is manufactured by Nokia Mobile Phones Ltd., Finland. By means ofa PCMCIA-type Nokia Cellular Datacard, which is manufactured by NokiaMobile Phones Ltd., the mobile station can be connected to any portablePC which is provided with a PCMCIA card location. The PCMCIA card thusprovides the PC with an access point, which supports the protocol of thetelecommunication application used in the PC, for instance CCITT X.25 orInternet Protocol IP. Alternatively, the mobile station may directlyprovide an access point which supports the protocol used by the PCapplication. Furthermore, it is possible that the mobile station MS andthe PC are integrated into a single unit within which the applicationprogram is provided with an access point supporting the protocol used byit. An example of such a mobile station with an integrated computer isthe Nokia Communicator 9000, also manufactured by Nokia Mobile PhonesLtd., Finland.

Network elements BSC and MSC are known from a typical GSM network. Thearrangement of FIG. 1 includes a separate support node SGSN (ServingGPRS Support Node) of the GPRS service. This support node SGSN controlscertain operations of the packet radio service on the network side. Suchoperations comprise logging on and off the system by the mobile stationsMS, routing area updates of the mobile stations MS, and routing of datapackets to their correct targets. Within this application, the conceptof “data” should be understood broadly to cover any informationtransferred in a digital communication system. Such information cancomprise speech coded into digital form, data transmission betweencomputers, telefax data, short segments of program code, etc. The SGSNnode can be located at a BTS or at a BSC or at an MSC, or it can belocated separately from any of these elements. The interface between aSGSN node and the Base Station Controller BSC is called a GB interface.

In a GPRS system the term “group” is used to refer to a plurality ofmobile stations MS that are registered with a common InternationalMobile Group Identifier (IMGI). A message that is to be sent to allmembers of a group is called a “group message”. There can be open groupsand closed groups but the present invention is equally applicable toboth types of groups.

Two types of Point-To-Multipoint (PTM) services are defined in GPRS:

PTM-Multicast (PTM-M); and

PTM-Group (PTM-G).

For controlling transmissions of Point-To-Multipoint messages, a GPRSsystem typically comprises a Point-To-Multipoint Service Center, PTM-SC,also known as a PTM server.

PTM-M is a service for broadcasting data sent by a service provider overa geographical area, such as a city. The geographical area for which aPTM-M message is destined can be defined in the message from the serviceprovider to the PTM server PTM-SC. PTM-M service supports neither securedelivery nor security of data. This means that anyone sending messagesusing PTM-M service cannot be sure that the message will be delivered tothe receiving party. Due to the nature of the PTM-M service, anyone canlisten to it, and therefore, no data security is guaranteed.

In PTM-G transmission the delivery can optionally be secure and the dataare ciphered. (The current understanding is that the SGSN establishes aconnection similar to Point-To-Point (PTP) with each mobile stationwhich has a group registration in the geographical area). The securityof the transmission is the same as over the Point-To-Point connection.

Thus in prior art packet radio systems, the same PTM-G message will besent X times if there are X subscribers in a certain routing area. Itcan be noted that with the increasing use of PTM-G transmissions, alarge amount of traffic will be generated over the air interface ofpacket radio systems.

DISCLOSURE OF THE INVENTION

Based on the foregoing description, it is an object of the presentinvention to create a method and an arrangement for PTM-G messagetransmission that do not suffer from the above drawbacks. The object ofthe invention will be achieved with a method and an arrangement whichare characterized by what is disclosed in the appended independentclaims. Advantageous embodiments of the present invention will bepresented in the dependent claims.

The invention is based on the idea of sending a PTM-G group message overa combination of broadcast and point-to-point (PTP) connections. Whenthe message is broadcast, it is sent in a way similar to the way thePTM-M message is sent (i.e. broadcast) but in a ciphered form. Aselection algorithm will be used to determine the optimal transmissionmode (broadcast or point-to-point transmission). Broadcast transmissionwill be used if it is determined that this mode of transmission loadsthe network less than point-to-point transmission. In a simpleembodiment of the algorithm, the number of those group members in thedestination area of the group message that have not yet received thegroup message will be compared to a predetermined (not necessarilyfixed) threshold. If the number of these remaining group members exceedsthis threshold, broadcast transmission will be used, possibly with a fewrepetitions. Unlike the current PTM-M recommendations, the group messagewill be preferably acknowledged by the mobile stations that receive themessage correctly. Then the group message can be sent to the remainingmobile stations via PTP connections like in prior art methods andsystems.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described by means of preferredembodiments with reference to the accompanying drawing, in which:

FIG. 1 is a block diagram illustrating some of the elements of a packetradio system to which the invention relates;

FIG. 2 is a signalling diagram which illustrates signalling betweencertain network elements in a packet radio system;

FIG. 3 is a flow chart which illustrates transmission of a groupmessage.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to FIG. 2. In step 2-1, when a mobile station MSjoins a group, it has to activate a group routing context in the SGSN. Arouting context is a context established in different GPRS nodes whichallows a packet addressed to the MS to reach it. A group routing contextwill allow the group message to be routed to the group members by thePTM-SC and the SGSN. In one possible implementation, it consists ofparameters indicating to the PTM-SC which SGSN (which has at least onegroup member) should receive the group message, and to each SGSN whichgroup member MS should receive the group message. Then the SGSN knowsthat the mobile station MS is a member of the group, and the SGSN cansend the group message to it. In step 2-2 the SGSN determines if a grouprouting context has already been activated for this group in the PTM-SC.If a group routing context has been activated for this group (i.e. thereis already at least one group member registered in the SGSN), theprocess advances to step 2-5. However, if a group routing context hasnot been activated in the PTM-SC, the SGSN sends an Activate GroupContext request to the PTM server PTM-SC in step 2-3. In step 2-4 thePTM-SC returns an acknowledgment of the activation request.

In step 2-5 an acknowledgment is sent to the mobile station MS. In thisanswer message Routing Context Activated, the SGSN will include thegroup ciphering key (a key common to the entire group) which may havebeen sent by the PTM server. (Alternatively, this key could also begenerated by the SGSN).

This key can be changed periodically. In the example shown in FIG. 2,the PTM server sends a message called Change Group Ciphering Key to theSGSN. This message includes the new ciphering key. This instructs theSGSN to send a Change Group Key message to every mobile station MS ofthe group (typically over a PTM connection ciphered with the individualciphering key of the MS). This key is different for every group. Steps2-10 through 2-13 describe the process of changing the group cipheringkey. Alternatively, the change of a group ciphering key can beacknowledged by the mobile station MS to the support node SGSN and bythe SGSN to the PTM server PTM-SC.

It is only for the purpose of keeping FIG. 2 clear and simple that thesteps 2-10 through 2-13 are drawn after (=below) the other steps. Inreality, the PTM-SC can change the group ciphering key at any time. InFIG. 2 this means that the group of steps consisting of steps 2-10through 2-13 could be located anywhere in relation to the other steps2-1 through 2-9.

It is possible that a mobile station MS does not have the right groupciphering key either because it missed the key or because it does nothave enough memory to store the key. Such a mobile station will not beable to listen to group messages when they are broadcast. In this case,according to an embodiment of the present invention, the support nodeSGSN will send the message using a Point-to-Point connection.Alternatively, if a mobile station MS cannot store the group cipheringkey (due to lack of memory), it could inform the SGSN so that the SGSNknows that this mobile station MS cannot receive broadcast messagesusing the group ciphering key. Thus the SGSN does not have to send thischange key message. It can also mark certain mobiles as not having thekey. The SGSN might not take such mobile stations into account when itchecks the criteria for broadcasting as it knows that such mobilestations can not listen to the broadcast messages.

Steps 2-6 through 2-9 relate to the way a group message is sent andacknowledged. In step 2-6, the PTM server PTM-SC will send the PTM-Gmessage to all the support nodes SGSN that have reported having groupmembers and cover the geographical area where the group message shouldbe sent. Then in step 2-7, the support node SGSN will decide for eachrouting area how the group message should be sent. The SGSN can useeither broadcast or point-to-point transmissions. The SGSN tries toestimate which transmission mode loads the critical parts of the networkleast (especially the air interface of a packet radio system).

If the criteria for broadcasting are fulfilled, the SGSN will broadcastthe group message (ciphered with the group ciphering key). According toa preferred embodiment of the invention, the group message is broadcastusing a predetermined schedule, such as broadcasting the group message ntimes at predetermined intervals.

If the criteria for broadcasting are not fulfilled, the SGSN will sendthe group message over a PTP connection to each group member. Inperforming the analysis to determine whether or not the criteria forbroadcasting the message are fulfilled, the SGSN can use at least someof the following parameters: 1) The number of group members in therouting area able to receive broadcast messages. 2) The quality ofservice requested. 3) The number of cells in the routing area. 4) Thenumber of group members currently transmitting (as they do not need tobe paged and they have a higher probability of missing a broadcastmessage, which might affect the load calculation). 5) The fact that thesame group message has just been broadcast (which decreases theprobability that the mobile station MS which missed it will receive it).6) The probability of a group member receiving a broadcast messagecorrectly. It is only for the simplicity and clarity of FIG. 2 that allthese transmission modes and their combinations have been combined intoa single arrow 2-7. Advantageous algorithms for selecting the optimaltransmission mode will be described later, in connection with FIG. 3.

In step 2-8, the mobile stations MS that receive correctly the entiregroup message will acknowledge it. Alternatively, the group members whodid not receive the message correctly could send a negativeacknowledgment to indicate that they missed the group message at leastpartially. It is useful to send this negative acknowledgment after atime-out long enough to ensure that no other broadcast transmissions arescheduled. It could also be useful in a GPRS system to avoid having topage these mobiles. If a PTP connection is used, the acknowledgment is anormal feature of a PTP transmission in a GPRS system. Each groupmessage should be identified in a unique way so that the mobile stationMS can delete the group message it has already received in a previoustransmission. The same group message should not be acknowledged morethan once. One way to indicate to a mobile station MS that a broadcastmessage has to be acknowledged, is to indicate it in the coding of theIMGI.

In step 2-9, the support node SGSN acknowledges the group message to thePTM server PTM-SC. Alternatively, it can indicate such parameters as thenumber or the percentage of subscribers who received the messagecorrectly, or their identity (IMSI).

As soon as the support node SGSN has broadcast the message once andrepeated it the scheduled n times, it will receive an acknowledgmentfrom the mobile stations MS that received the message correctly. Thenthe support node SGSN can calculate the number of remaining groupmembers in each routing area. On the basis of this number, the supportnode SGSN has to decide again how to send the PTM-G message. It willsend it in broadcast form if this number is higher than a predeterminedthreshold.

When the remaining number of group subscribers (those who have notreceived the group message) is smaller than this threshold, the supportnode SGSN establishes an individual connection with each group member(similar to a normal PTP connection) and sends them the message usingtheir own ciphering keys. An upper limit should be defined to thebroadcast transmissions in order to prevent the SGSN from endlesslybroadcasting the message to group members who are not reachable.

Alternatively, the support node SGSN indicates to the PTM server PTM-SCthe mobile stations MS which were not reachable. According to analternative embodiment of the present invention, the message is sentwhen the mobile stations MS become reachable before a certain time-out.In a way, this procedure is analogous to the short message service (SMS)in a GSM system where the HLR informs the PTM server PTM-SC that the MSis now reachable.

Criteria Establishment

Referring now to FIG. 3, a flow chart illustrating the decision makingprocess will now be described. In step 3-1 the support node SGSNreceives from the PTM server PTM-SC a group message including theidentity of the group (IMGI) and the geographical area to which thisgroup message should be sent. In step 3-2 the support node SGSN checkseach routing area of this geographical area in order to find out whetherat least one group member is registered or not. If none are registered,the SGSN determines in step 3-3 that there are no messages to send tothis routing area. However, if X (X being at least equal to 1) groupmembers are registered, the process advances to step 3-4 where thesupport node SGSN applies an algorithm to determine the optimaltransmission mode (broadcast or point-to-point) for this number X. Asuitable algorithm will be described below in greater detail. If theSGSN determines that the criteria for broadcasting are fulfilled, theprocess advances to steps 3-5 and 3-6 where the SGSN broadcasts themessage with n repetitions and waits for the acknowledgments from themobile stations MS. The process repeats the loop consisting of steps3-4, 3-5 and 3-6 until the SGSN determines that a sufficient number ofmobile stations MS have acknowledged reception of the message (or theloop has been repeated the maximum number of times) and the criteria forbroadcasting are no longer fulfilled. At this point, the processadvances to step 3-7 because the SGSN can make better use of theavailable resources by sending the group message over point-to-pointconnections to those mobile stations MS that have not acknowledgedreception of the message. In step 3-8 the support node SGSN can send areport to the PTM server indicating the quality of service (QoS) usedand optionally a list of subscribers that have received (or missed) themessage.

One possible test for determining the optimal transmission mode will nowbe explained in greater detail. In the discussion, the followingnotation will be used:

“C” denotes the number of cells in the routing area.

“X” denotes the number of group members in the routing area.

“X_(c)” denotes the number of group members per cell (X_(c)=X/C).

“S(paging)” denotes the size of the paging message per cell.

“S(PTM-G)” denotes the size of the PTM-G message to be sent.

“S(ack)” denotes the size of the acknowledgment.

“p” denotes the probability of receiving a broadcast message correctly(this parameter depends on the size of the message, the number n of thetimes the message is broadcast, radio conditions, etc.)

If the message is transmitted over individual (=PTP) connections to allmobile stations of the group, the load generated can be calculated from:

L(I)=C*X*S(paging)+X*S(PTM-G)  [1]

If, however, the message is broadcast with n repetitions to all mobilestations of the group before being sent with PTM, the load generated canbe calculated from:

L(B)=n*C*S(PTM-G)+X*p*S(ack)+C*X*(1−p)*S(paging)+X*(1−p)*S(PTM-G)  [2]

The criterion for broadcasting the message is: L(B)<L(I), or:

n*C*S(PTM-G)+X*p*S(ack)<p*C*X*S(paging)+p*X*S(PTM-G)  [3]

As we can assume that X*p*S(ack)<p*C*X*S(paging) is always true, thecriteria will be fulfilled if:

n*C<X*p  [4]

If we use X_(C) which has been defined as the number of groupsubscribers per cell, i.e. C*X_(C)=X, an even simpler formula can beobtained:

n<p*X _(C)  [5]

The value of p depends on many parameters, and it is difficult to knowtheir values with good accuracy. These parameters include the number nof repetitions, the size of the message, and the transmissionconditions. Thus it is difficult to calculate the value of p accurately,and in practice it can be determined experimentally by keeping track ofthe acknowledgments to previous messages. Ideally, the SGSN shouldmaintain for each routing area an updated value of p which is calculatedusing the previous value measured. Let us assume for example that n=2,p=75% and X_(C)=4. These parameters fulfil the criteria because 2<3. Inother words, the load generated by the method according the invention ismore than ⅓ smaller than the load generated by prior art transmissionmethods. This advantage is most noticeable at the air interface inhigh-density areas, because it is very difficult to increase thecapacity of the air interface. Very often, increasing the capacity ofthe air interface requires installations of additional base stationsand/or reorganization of the frequency re-use pattern of severalneighbouring base stations.

Alternative Embodiments

The support node SGSN or the PTM server PTM-SC can optionally dividelong group messages into smaller frames. The SGSN can send these framesto the mobile stations MS, as already described in connection with agroup message. Each frame should be uniquely identified. Alternatively,an indication that the frames are parts of a longer message could beincluded. For example, the frames could be numbered and the first andthe last frame could be indicated. The MS could also acknowledge manyframes in the same acknowledgment message. It could use a timer toensure that no other broadcast retransmission are coming before sendingan acknowledgment. According to an embodiment of the present invention,the SGSN only has to resend the frames that the mobile station failed toreceive correctly. These frames could be resent over a broadcastconnection (if many mobile stations MS missed the same frames) or overPTP connections. A surprising benefit of this embodiment is that byacknowledging other frames the MS also indicates its cell. Thus, thereis no need to page the MS in order to send the PTP message.

According to yet another alternative embodiment of the invention, thePTM-SC knows which group members are registered in each routing areaunder each support node SGSN. Then the criteria for the transmission ofthe message could be defined in the PTM-SC instead of the SGSN. Thisembodiment preserves a simple SGSN which has to create a routing contextfor each group member in the PTM-SC and to forward all theacknowledgment to the PTM-SC. The PTM-SC will inform the SGSN of whichpacket to broadcast in which routing area and which packet to send overa PTP connection to which group members. The PTM-SC could also checkwhether the subscriber has a right to join a group.

With the increasing use of the PTM-G service, the method and arrangementof the present invention will save a considerable amount of resources onthe air interface of a packet radio system. The invention will be foundespecially useful in areas where radio traffic is intense, such as citycenters, premises of major corporations, etc. A considerable amount ofresources can be saved if the number of subscribers per cell istypically five or more.

No hardware modifications are necessary in the network infrastructure.Instead, all modifications required for implementing the invention canbe realized as additional or upgraded software routines of the SGSNand/or PTM server. Thus the invention can be implemented relativelyeasily in the network elements.

The additional memory required in mobile stations MS is limited to asmall amount which is sufficient for storing additional ciphering keys.Even if the mobile station lacks this additional memory, the user is notprevented from using the group service. The user will only be preventedfrom listening to the broadcast messages.

The invention has been described by means of its preferred embodiments.However, the specifications for packet radio technology in general andGPRS in particular are developing rapidly. Such developments may requireadditional modifications to the invention. Therefore, all words andexpressions should be interpreted broadly, and they are intended fordescribing rather than limiting the invention defined in the appendedclaims.

What is claimed is:
 1. A method for transmitting group messages in apacket radio network comprising at least one support node (SGSN) and atleast one group comprising a plurality of group members (MS), and atleast one defined area, said support node (SGSN) knowing the identity ofsaid group members (MS) in said area, wherein said network comprises:broadcast connections between said support node (SGSN) and saidplurality of group members (MS); and point-to-point connections betweensaid support node (SGSN) and each one of said plurality of group members(MS); characterized in that said method comprises performing thefollowing steps at least once: (i) keeping track of the group members(MS) that have not received said group message and determining whetheror not their number exceeds a predetermined threshold; (ii) in responseto a positive determination in the previous step, transmitting saidgroup message over a broadcast connection; and in response to a negativedetermination, transmitting said group message over a point-to-pointconnection to said group members (MS) that have not received said groupmessage.
 2. A method according to claim 1, further characterized in thatsaid broadcast transmission is repeated a fist predetermined number oftimes at predetermined intervals.
 3. A method according to claim 1,further characterized in that said predetermined threshold is formed bycomparing the load generated by broadcast connections (L(B)) with theload generated by point-to-point connections (L(I)).
 4. A methodaccording to claim 1, further characterized in that during saidbroadcast connections at least some of the messages are ciphered using agroup ciphering key common to said group and during said point-to-pointconnections at least some of the messages are ciphered using anindividual ciphering key.
 5. A method according to claim 4, furthercharacterized in that said support node (SGSN) is arranged to distributesaid group ciphering key using said ciphered point-to-point connections.6. A method according to claim 1, further characterized in that saidsupport node (SGSN) is arranged to keep track of said group members (MS)who are currently unable to receive said group message over saidbroadcast connection.
 7. A method according to claim 6, furthercharacterized in that said support node (SGSN) is arranged to transmitsaid group message over said point-to-point connection to said groupmembers (MS) who are currently unable to receive said group message oversaid broadcast connection.
 8. A method according to claim 1, furthercharacterized in that said support node (SGSN) is arranged to keep trackof the identity of said group members (MS) who received said groupmessage and to forward this information to another network node(PTM-SC).
 9. A method according to claim 1, further characterized inthat said support node (SGSN) is arranged to keep track of the identityof said group members (MS) who did not receive said group message and toforward this information to another network node (PTM-SC).
 10. A methodaccording to claim 8, further characterized in that said other networknode (PTM-SC) is arranged to transmit said group message to said groupmembers (MS) who did not receive said group message when these groupmembers (MS) become reachable.
 11. A method according to claim 1,characterized in that the number of times said steps (i) through (ii)are repeated is limited to a second predetermined number.
 12. A methodaccording to claim 1, further characterized in that said firstpredetermined number (n) is calculated separately for each broadcasttransmission.
 13. A method according to claim 1, further characterizedin that said group message is transmitted as frames, each framecomprising a subset of said group message, and each frame beingacknowledged individually by each one of said group members (MS) thathas received said frame correctly.
 14. A method according to claim 1,further characterized in that said group message is transmitted asframes, each frame comprising a subset of said group message, and allframes of the same group message being acknowledged with a commonacknowledgment by each one of said group members (MS) that has receivedsaid frame correctly.
 15. A method according to claim 1, furthercharacterized in that each one of said frames is retransmittedindependently of the other frames, in response to a determination thatit has not been received correctly by all group members (MS). 16.Network element (SGSN, PTM-SC) for a packet radio network comprising atleast one support node (SGSN) and at least one group comprising aplurality of group members (MS), and at least one defined area, saidsupport node (SGSN) knowing the identity of said group members (MS) insaid area, wherein said network comprises: broadcast connections betweensaid network element (SGSN, PTM-SC) and said plurality of group members(MS); and point-to-point connections between said network element (SGSN,PTM-SC) and each one of said group members (MS); characterized in thatsaid network element (SGSN, PTM-SC) comprises: means for keeping trackof the group members (MS) that have not received said group message andmeans for determining whether or not their number exceeds apredetermined threshold; means for transmitting said group message overa broadcast connection in response to a positive determination by saiddetermining means; and means for transmitting said group message to saidgroup members (MS) over a point-to-point connection in response to anegative determination by said determining means.
 17. Network element(SGSN, PTM-SC) according to claim 16, further characterized in that saidnetwork element comprises a processor arranged to execute softwareroutines and that said means are implemented as software routines.